UMNSVP Composite Chassis Design
Introduction The University of Minnesota Solar Vehicle Project has been building their chassis out of prefabricated composite panels since 1995, and over the course of 8 cars, has refined the process considerably. Although our box-beam chassis design is not particularly complex, there appear to be several important facets of our design and construction process that are unique to our project. However, there's nothing particularly secret about it, so this document aims to present our process in a clear fashion. Step One: Materials and Construction Technique Testing The first step in building any composite structure should be to test your materials and construction techniques to failure. When designing a structure that is composed of glued composite panels, is is crucial to know four values in particular: #The bending strength of the panels (verify manufacturer specs) #The maximum tensile force the glue joint can withstand per unit length #The maximum bending moment the glue joint can withstand per unit length #The pullout strength in sheer (in-plane forces) of the method of bolting brackets to panels Note that we do NOT generally care about the pullout strength of bolts in the out-of-plane direction. This will be discussed later. You need to perform these tests to verify to yourself and the team that you know what you are doing, and your chassis will not fail. The inspectors also love to hear that you have empirically tested your parts to failure. The following diagram shows a standard "T-joint" between two panels. I'm sure you can dream up all sorts of brackets to hold the bottom panel in place while applying force in the vertical and horizontal direction to the upright panel, in order to perform tests #2 and #3. You will note in the above diagram that the radii of the glue fillets are called out. "Ri" is mostly a pipe dream, just get a healthy amount of glue in there. "Re", on the other hand, can be closely controlled. A tongue depressor with the correct radius tip is run along the joint after the glue is laid down; pressing the glue into the corner, shaping the fillet, and removing excess glue in one stroke. Make heavy use of masking tape to keep glue off of places it shouldn't go. The exterior glue fillet provides the large majority of the strength of this T-joint, so take care to do the fillet properly. Glue selection is likely the most crucial choice to make here. Many teams wrap fabric across joints like this for extra strength. With the correct glue, this extra (messy, weight-adding) process is unnecessary. At Minnesota, we use a 3M dual-part adhesive called DP-460NS. When testing the maximum bending moment a glued T-joint can sustain, we delaminate the panels before glue failure occurs. When testing the maximum tensile strength of the glue joint, the panels either delaminate or fail in sheer before the glue fails. If the panels are strong enough, and they fail before the joints do, the joints are certainly more than strong enough. Performing test #4 is quite simple. Take a sample panel, and attach a bolt through the panel at either end. Using one bolt, hand the panel from something sturdy. Devise a method to suspend weight from the lower bolt. Add weight until the part fails. See below for how we tested our bolting method for our 2010 car, Centaurus 2: File:UMNSVP composite panel sheer test 1.JPG|Initial test setup File:UMNSVP composite panel sheer test 2.JPG|Final setup, ready to add weight File:UMNSVP composite panel sheer test 3.JPG|You may have to add a LOT of weight... Each of those buckets pictured is ~80lbs of sand. In the end, we stuck another bucket onto either end of the 4x4, and both myself and Sam hung off the ends. No failure was observed, and no creaking or cracking was heard. At this point, we had loaded 1130lbs onto the panel, and were at the load rating of the chains, so we stopped. At a later date, one of our members took the panel to a fancy computerized tensile test machine, where the bolts ripped out of the panel at ~1600lbs of tension. Ideally, you would use an instrumented tensile test machine for the all tests listed above, but the above pictures illustrate what can be done with cheap supplies that you may have on hand. Grommets Some of you may have noticed that 1600lbs failure; yes, we could hang 4 solar cars off of a single bolt. The secret is the aluminum aircraft floor panel fasteners ("grommets") that we install into the panels. File:UMNSVP TYE5319 Grommet 1.JPG File:UMNSVP TYE5319 Grommet 2.JPG These grommets are made by The YOUNG ENGINEERS, Inc. They are intended to be used with 100° countersunk AN bolts, although standard hex-head bolts can be used if a washer is placed between the head of the bolt and the countersink. TYE makes a wide variety of these grommets for different diameter bolts, with different diameter flanges, and for different panel thicknesses. Those pictured above are for a 3/16" bolt, have a 1" flange, and are for 0.5" thick panels. Minnesota has traditionally used TYE's aluminum grommets, however, they have recently come out with a line of Torlon plastic grommets that are supposedly lighter and just as strong. Minnesota has had good luck getting TYE to donate grommets in the past. Call their central phone number (949-581-9411) and ask for Sam Frias, the Engineering Manager. Almost all of the strength of the grommet comes from the glue that is used to attach the flange of the grommet to the face of the panel. To illustrate: for comparison, a panel identical to the one pictured above was prepared without gluing the grommets. The panel failed at approximately 150lbs! To install, drill a hole in the location you wish to bolt through. After drilling, clean the surface around the hole thoroughly. Apply glue to the flanges of the grommet, and loosely insert them. The grommet halves are press-fit together, use a bolt that you don't mind getting glue all over to tighten them together. Don't overtighten; if you crush the panel, you will greatly weaken the part! Clean off any excess glue, and allow the glue to fully set before applying force to the grommet. Glue choice is up to you; Minnesota prefers to use the DP-460NS that the rest of the chassis is held together with. Build a Full-Scale Mockup Category:UMNSVP